Needle‐free cervical cancer treatment using helical multishield intracavitary rotating shield brachytherapy with the ¹⁶⁹Yb Isotope

Abstract

Purpose: To assess the capability of an intracavitary 169 Yb-based helical multishield rotating shield brachytherapy (RSBT) delivery system to treat cervical cancer. The proposed RSBT delivery system contains a pair of 1.25 mm thick platinum partial shields with 45°and 180°emission angles, which travel in a helical pattern within the applicator. Methods: A helically threaded tandem applicator with a 45°tandem curvature containing a helically threaded catheter was designed. A 0.6 mm diameter 169 Yb source with a length of 10.5 mm was simulated. A 37-patient treatment planning study, based on Monte Carlo dose calculations using MCNP5, was conducted with high-risk clinical target volumes (HR-CTVs) of 41.2-192.8 cm 3 (average AE standard deviation of 79.9 AE 35.8 cm 3 ). All patients were assumed to receive 25 fractions of 1.8 Gy of external beam radiation therapy (EBRT) before receiving 5 fractions of high-dose-rate brachytherapy (HDR-BT). For each patient, 192 Ir-based intracavitary (IC) HDR- BT, 192 Ir-based intracavitary/interstitial (IC/IS) HDR-BT using a hybrid applicator with eight IS needles, and 169 Yb-based RSBT plans were generated. Results: For the IC, IC/IS, and RSBT treatment plans, 38%, 84%, and 86% of the plans, respectively, met the planning goal of an HR-CTV D 90 (minimum dose to hottest 90%) of 85 Gy EQD2 (a/ b = 10 Gy). Median (25th percentile, 75th percentile) treatment times for IC, IC/IS, and RSBT were 11.71 (6.62, 15.40) min, 68.00 (45.02, 80.02) min, and 25.30 (13.87, 35.39) min, respectively. 192 Ir activities ranging from 159.1-370 GBq (4.3-10 Ci) and 169 Yb activities ranging from 429.2-999 GBq (11.6-27 Ci) were used, which correspond to the same clinical ranges of dose rates at 1 cm off-source-axis in water. Extra needle insertion and planning time beyond that needed for intracavitary-only approaches was accounted for in the IC/IS treatment time calculations. Conclusion: 169 Yb-based RSBT for cervical cancer met the HR-CTV D 90 goal of 85 Gy in a greater percentage of the patients considered than IC/IS (86% vs 84%, respectively) and can reduce overall treatment time relative to IC/IS. 169 Yb-based RSBT could be used to replace IC/IS in instances where IC/IS treatment is not available, especially in instances when HR-CTV volumes are ≥30 cm 3 .

A novel minimally invasive dynamic‐shield, intensity‐modulated brachytherapy system for the treatment of cervical cancer

“…Hopfensperger et al reported on a rotating helical multi-shielded tandem applicator capable of housing a pair of 1.25 mm thick platinum shields and requires a custom 0.6 mm diameter 169 Yb source for appreciable dose modulation. 41 The authors did not report statistical significance for differences in dose but showed that 86% of their plans met constraints compared to 84% for IC/IS-BT plans. By comparison, our IMBT system with 169 Yb led to 100% of the evaluated cases meeting dose constraints, an 11% improvement relative to non-IMBT.…”

“…Due to its design capabilities of housing a 5.4 mm diameter shield, even 192 Ir‐based IMBT can improve tumor coverage and OAR sparing. Hopfensperger et al reported on a rotating helical multi‐shielded tandem applicator capable of housing a pair of 1.25 mm thick platinum shields and requires a custom 0.6 mm diameter 169 Yb source for appreciable dose modulation 41 . The authors did not report statistical significance for differences in dose but showed that 86% of their plans met constraints compared to 84% for IC/IS‐BT plans.…”

On the impact of absorbed dose specification, tissue heterogeneities, and applicator heterogeneities on Monte Carlo‐based dosimetry of Ir‐192, Se‐75, and Yb‐169 in conventional and intensity‐modulated brachytherapy for the treatment of cervical cancer

“…To fully transition to MDBCAs, sitespecific dosimetric data must be collected so that it may be available for correlation with known clinical outcomes. Additionally, considering the growing interest in IMBT, where high Z metal shields are introduced and radionuclides emitting lower energy photons than 192 Ir are favorable, [4][5][6]30 understanding how MBDCAs and the various degrees of approximations impact dosimetry are important for guiding professional societies establish guidelines for IMBT. In this work, we note that the trends of various dose reporting schemes are consistent and between conventional brachytherapy and IMBT within 1% for non-bone tissues and therefore conclusions drawn from conventional brachytherapy generalize well for IMBT.…”

“…An active area of research and innovation in brachytherapy is the use of static and dynamic high Z metal shields for intensity-modulated brachytherapy (IMBT). [3][4][5][6] In addition to the ubiquitous 192 Ir high-dose rate (HDR) brachytherapy source with an average photon energy (E γ,avg ) of 380 keV, alternative lower energy sources such as 75 Se (E γ,avg = 210 keV) and 169Yb (E γ,avg = 93 keV) have been investigated due to their potential to maximize the modulation capabilities of IMBT due to their lower photon energies. [5][6][7][8] The impact of tissue/applicator heterogeneities and dose reporting schemes (D w,w vs D m,m ) on gynecologic brachytherapy have only been investigated for high-energy sources.…”

Optimization in treatment planning of high dose‐rate brachytherapy — Review and analysis of mathematical models

“…They show reductions in dose to urethra with on average 13.3%. In a study on cervical cancer 119 they conclude that IMBT without interstitial needles gives similar results as conventional HDR BT with needles. Similar improvements are seen in another cervical cancer study, 120 either as sparing of OAR or a reduction in the number of needles.…”